Monoclonal antibody drugs bind to specific antigens on target cells. In some cases, once attached to their antigens, the drugs stimulate the patient's immune system to attack the labeled cells. In other cases, the mechanism of therapeutic action that follows binding remains unknown. Rituximab, a monoclonal antibody that targets a specific antigen (CD20) on B-cells has become a common therapeutic for patients with B-cell lymphoma; it is also being used to treat systemic lupus erythematosus (SLE) and other autoimmune disorders. There are additional monoclonal antibody drugs in development that target B-cell antigens, including CD20 and other surface expressed antigens.